At some point, all people of any level of faith question whether something they have done and consider a sin is, will be or can be forgiven by God. We've all done and said things we regret, some of us with consistent regularity. We may reason that surely God will not forgive the same sins we commit over and over.
There is some pretty consistent theology for that line of reasoning. When you look at God's reactions to sin committed by his own people in the Old Testament, you can end up with a conclusion that you are so bad God is fed up with your continuing rebellion against his word. I have had this discussion many times with some truly good people who never feel "good enough". They have assumed that God's grace isn't good enough or powerful enough to overcome their sins, so at some point they adopted an attitude of "what's-the-use".
Paul addresses this attitude of inevitability - "I can't do it anyway so why go through the bother of even trying!" At Romans 4, he uses Abraham as the prime example of God's forgiveness and actually quotes David, who wrote, "Blessed are those whose lawless deeds are forgiven and whose sins are covered, blessed is the man against whom the Lord will not count sin." (Psalm 32:1-2) Following this quote Paul goes into a long explanation of how the grace of God worked for Abraham AND for all who have the faith of Abraham. God did not declare Abraham righteous because he was circumcised but because he was a man of faith. Paul then draws this conclusion: "Therefore, since we have been justified by faith, we have peace with God through our Lord Jesus Christ."
Sin is always a transgression of the will of God and its always a serious thing to ignore God's will. However, because of Jesus Christ, we "have peace with God" and can always know that God's grace is greater than any sin we can commit. That doesn't mean His grace is unconditional forgiveness, but if we adopt the faith of Abraham and David, there is no sin that goes uncovered. HALLELUJAH!